Reversible heat-sensitive recording medium

ABSTRACT

The reversible heat-sensitive recording medium having a heat-sensitive recording layer capable of reversibly changing the transparency thereof with organic low molecular substances dispersed in an organic macromolecular resin provided on a support is characterized by using at least one long chain alkyl-containing compound having a melting point of 50°-100° C. and at least one saturated aliphatic bisamide having a melting point of not less than 110° C. as the organic low molecular substance in a ratio of 98:2 to 80:20. A plasticizer may be contained in the organic macromolecular resin in a ratio of 1 to 10% by weight, based on the total solid of the heat-sensitive recording layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a reversible heat-sensitive recordingmedium capable of reversibly recording and erasing an image by theapplication of heat.

2. Description of the Related Art

Japanese Patent early Publication Nos. 154198/1980 and 257883/1987suggest reversible heat-sensitive recording media, according to whichdue to the difference of the temperatures upon heating, the transparencyafter being cooled can be reversibly changed, whereby images can bereversibly recorded thereon and erased therefrom. These recording mediahave a layered structure, including a layer, in which an organic lowmolecular substance such as a higher fatty acid is dispersed in anorganic macromolecular resin such as a vinyl chloride-vinyl acetatecopolymer in a fine particle form, as a heat-sensitive recording layerprovided on a support.

The relation between the transparent state of these reversible recordingmedia and the heat history given therefor is shown in FIG. 1. FIG. 1 isa constructional diagram showing the relation between the temperature ofreversible heat-sensitive recording media and the transparency. Theopaque (milky white state) portion can become transparent by heating itto a given temperature in the range of T1 to T2, and then graduallycooling it down to room temperature TR [(1)→(3)→(5)→(6)]. Conversely, inorder to turn a transparent portion into an opaque state, thetransparent portion may be heated to a temperature exceeding T3, andthen gradually cooled down to room temperature [(6)→(5)→(4)→(2)→(1)]. Inthese cases, the transparent state or opaque state is stably maintainedat room temperature TR as is.

In the conventional process, the range of the heating temperature,T1-T2, for an opaque portion becoming transparent (temperature range toachieve transparent state: TW) is very narrow and cannot be controlledin a desired range. Consequently, for a reversible heat-sensitiverecording medium in an .opaque state becoming transparent, it isrequired to control the temperature strictly and, thus, the practicaluse of the conventional process entails great difficulty.

If a reversible heat-sensitive recording medium is heated for asufficient duration of time by using a heating medium having asufficient heat capacity, e.g., an oven or a heat block, in the casewhere the reversible heat-sensitive recording medium in an opaque stateis to become transparent, the temperature range to achieve transparentstate: TW (referred to as clarifying temperature" hereinafter) may notbe so wide. However, if the heating media for the medium becoming opaquehas an insufficient heat capacity, or if it cannot be heated for asufficient period, e.g., in the case of the heat application by means ofa thermal head or laser for a period of several milliseconds, then theclarifying temperature range TW should be set sufficiently broad,because a temperature gradient toward the thickness direction of thereversible heat-sensitive recording medium is caused upon being heated,resulting in the medium not being able to be heated to a uniformtemperature. Nevertheless, the conventional reversible heat-sensitivemedia have a very wide range of the clarifying temperature, thus, it isdifficult to use a thermal head, etc. as a heating medium for erasingimages.

Yet, when a white opaque portion is erased to allow the media to becometransparent by means of a heating medium which has an insufficient heatcapacity or which cannot be heated over a sufficient period of time, theconventional reversible heat-sensitive media have the problem of nothaving sufficient erasing ability.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to solve the problemsof the prior art and to provide a reversible heat-sensitive recordingmedium having an extended clarifying temperature range TW, i.e., capableof controlling a temperature with ease and having goodopaque-portion-erasing characteristics.

In one aspect of the present invention, there is thus provided areversible heat-sensitive recording medium having a heat-sensitiverecording layer capable of reversibly changing the transparency thereofwith organic low molecular substances dispersed in an organicmacromolecular resin provided on a support, at least one long chainalkyl-containing compound having a melting point of 50°-100° C. and atleast one saturated aliphatic bisamide having a melting point of notless than 110° C. being used as the organic low molecular substance in aratio of 98:2 to 80:20.

In another aspect of the present invention, there is provided areversible heat-sensitive recording medium having a heat-sensitiverecording layer capable of reversibly changing the transparency thereofwith organic low molecular substances dispersed in an organicmacromolecular resin provided on a support, at least one long chainalkyl-containing compound having a melting point of 50°-100° C. and atleast one saturated aliphatic bisamide having a melting point of notless than 110° C. being used as the organic low molecular substance in aratio of 98:2 to 80:20, and the heat-sensitive recording layercontaining a plasticizer in a ratio of 1 to 10% based on the total solidcontent in the heat-sensitive recording layer.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a constructional diagram showing the relation between thetemperature of reversible heat-sensitive recording media and thetransparency.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The construction of the reversible heat-sensitive recording mediumaccording to the present invention will now be described.

The supports which can be used are those in which a color-coated layeris provided on a front or back surface of a synthetic resin film, thesynthetic films with a color pigment kneaded therein. Also usable aretransparent resin films, such as films made of polyvinyl chloride,polyester, polycarbonate, polyacetate, or polyimide, and those filmshaving a reflective layer made of a metal layer provided thereon.

Preferable for use in the heat-sensitive recording medium as the organicmacromolecular resin are those resins which have good transparency,excel in mechanical strength, and have good film-forming properties.Specific examples thereof include polyvinyl chloride, vinylchloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-maleicacid terpolymers, vinyl chloride-vinyl acetate-vinyl alcohol terpolymer,vinyl acetate-acrylate copolymers, vinylidene chloride-acrylonitrilecopolymers, polyester resins, polyamide resins, acrylic resins, siliconeresins, and the like.

The long chain alkyl-containing compound which is used as one of theorganic low molecular substances is generally termed "wax", andindicates the substance which is a solid at room temperature. Anycompound which contains a long chain alkyl group of approximately C₁₄-C₅₀ and has a melting point of 50° to 100° C. can be used, with anester, amide, or ketone having a long alkyl chain being preferable.Specific examples include, but are not limited thereto, stearylstearate, behenyl stearate, behenyl behenate, behenyl montanate, C₃₀-alcohol stearate, C₃₀ -alcohol behenate, C₅₀ -alcohol stearate, C₅₀-alcohol behenate, stearyl alcohol diester of eicosanedicarboxylic acid,etc. for the esters; palmitic acid amide, stearic acid amide, behenicacid amide, oleic acid amide, N-stearylstearic acid amide,N-oleylpalmitic acid amide, N-stearylerucic acid amide, N-stearyloleicacid amide etc. for the amides; and distearyl ketone, dibehenyl ketone,etc. for the ketones. These long alkyl chain-containing compounds may beused on their own or in combination with two or more compounds thereof.

The saturated aliphatic bisamides which can be used as the other organiclow molecular substance are those which have a melting point of not lessthan 120° C., preferably in the range of 130° to 150° C., and includeacid amides between a long chain saturated aliphatic acid and analkylene diamine as well as between a saturated aliphatic dicarboxylicacid and a saturated aliphatic amine. Specific examples include, but arenot limited to:

    ______________________________________                                        Ethylene bis-(stearic acid) amide                                                                    m.p. 143° C.                                    (C.sub.17 H.sub.35 CONH).sub.2 (CH.sub.2).sub.2                               Ethylene-bis(behenic acid) amide                                                                     m.p. 141° C.                                    (C.sub.21 H.sub.43 CONH).sub.2 (CH.sub.2).sub.2                               Hexamethylene-bis(stearic acid) amide                                                                m.p. 146° C.                                    (C.sub.17 H.sub.35 CONH).sub.2 (CH.sub.2).sub.6                               Hexamethylene-bis(behenic acid) amide                                                                m.p. 143° C.                                    (C.sub.21 H.sub.43 CONH).sub.2 (CH.sub.2).sub.6                               N,N'-distearyladipic acid amide                                                                      m.p. 144° C.                                    (C.sub.18 H.sub.37 NHCO).sub.2 (CH.sub.2).sub.4                               N,N'-distearyleicosanedicarboxylic                                                                   m.p. 128° C.                                    acid amide                                                                    (C.sub.18 H.sub.37 NHCO).sub.2 (CH.sub.2).sub.8                               N,N'-distearylsebacic acid                                                                           m.p. 138° C                                     (C.sub.18 H.sub.37 NHCO).sub.2 (CH.sub.2).sub.8                               N,N'-dilauryldodecanedicarboxylic                                                                    m.p. 138° C.                                    acid amide                                                                    (C.sub.12 H.sub.25 NHCO).sub.2 (CH.sub.2).sub.10                              N,N-dilauryleicosanedicarboxylic                                                                     m.p. 130° C.                                    acid amide                                                                    (C.sub.12 H.sub.25 NHCO).sub.2 (CH.sub.2).sub.18                              N,N-distearyldodecanedicarboxylic                                                                    m.p. 130° C.                                    acid amide                                                                    (C.sub.18 H.sub.37 NHCO).sub.2 (CH.sub.2).sub.10                              ______________________________________                                    

These saturated aliphatic bisamides may be used alone or as a mixture oftwo or more thereof.

In this case, if the saturated aliphatic bisamide has a melting point ofless than 120° C., there arises the problem where no substantialenlargement of the clarifying temperature range can be obtained.

In the present invention, the weight ratio of the above-mentioned longchain alkyl-containing compound to the aliphatic bisamide should bewithin the range of 98:2 to 80:20. If the proportion of the aliphaticbisamide which occupies the organic low molecular substance is less than2% by weight, there is no effect thereon upon the enlargement of theclarifying temperature range, while if it is higher than 20% by weight,no good contrast can be obtained.

For the formulation amounts of the organic low molecular substancesdispersed in the organic macromolecular resin, the sum of the long chainalkyl-containing compound and the aliphatic bisamide is preferably inthe range of 5 to 100 parts by weight, particularly 10 to 50 parts byweight, relative to 100 parts by weight of the organic macromolecularresin. If the total amount of the low molecular substances is less than5 parts by weight, the heat-sensitive recording layer does not changeinto an opaque state sufficiently, resulting in no good contrast beingobtainable. Conversely, if it is more than 100 parts by weight, thefilm-forming properties of the heat-sensitive recording layer change forthe worse.

Any suitable plasticizer can be used as the plasticizer in the case ofthe incorporation of a plasticizer in the present invention. Specificexamples are: phosphates, such as tributyl phosphate, tri-2-ethylhexylphosphate, triphenyl phosphate, and tricresyl phosphate; phthalates,such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate,di-n-octyl phthalate, diisooctyl phthalate, dicapryl phthalate,di-2-ethylhexyl phthalate, dioctyldecyl phthalate, diisodecyl phthalate,butyl benzyl phthalate, dibutoxy ethyl phthalate; monobasic fatty acidesters, such as butyl oleate, tetrahydrofurfuryl oleate, and glycerinemonooleate; dibasic fatty acid esters, such as dibutyl adipate,di-n-hexyl adipate, di-n-ethylhexyl adipate, di-2-ethylbutyl azelate,di-2-ethylhexyl azelate, dibutyl sebasate, di-2-ethylhexyl sebacate, anddicapryl sebacate; dihydrylic alcohol esters, such diethylene glycoldibenzoate, triethylene glycol di-2-ethylbutyrate, and triethyleneglycol di-2-ethylhexoate; oxyacid esters, such as methylacetylricinolate, butyl acetylricinolate, methoxyethyl acetylricinolate,butylphthalyl butylglycolate, and tri(2-ethylhexyl) acetylcitrate;chlorinated paraffin; chlorinated biphenyl, 2-nitrobiphenyl;dinonylnaphthalene; camphor, methyl abitate, and the like. They can beused singly or used in any suitable combination of two or more of them.

It is desired for the amount of the plasticizer added to fall in therange of 1 to 10% based on the total solid concentration in theheat-sensitive layer. If the amount of the plasticizer added in lessthan the above-mentioned range, there is no effect brought about by theaddition of the plasticizer, while if it is more than theabove-mentioned range, the concentration of white lettering isdecreased, thereby changing the contrast between the white letteringportion and the transparent skin for the worse.

The contrast can be enhanced if a light-reflective layer composed of athin metal film such as one made of aluminum is provided between theheat-sensitive layer and the support. In such a case, the differencebetween the reflection densities (Macbeth densities) of the opaqueportion and the transparent portion is preferably not less than 0.7.Taking these facts into consideration, the type and the amount of theplasticizer used can suitably be selected.

The heat-sensitive layer according to the present invention can beprovided on a support by applying or printing on a support a solution ofthe organic macromolecular resin which may contain the plasticizer, thelong chain alkyl-containing compound, and the saturated aliphaticbisamide; a dispersion of the organic macromolecular resin which maycontain the plasticizer, the long chain alkyl-containing compound, andthe saturated aliphatic bisamide; a mixture in which the organicmacromolecular resin which may contain the plasticizer, the long chainalkyl-containing compound, and the saturated aliphatic bisamide aremelted and mixed; or the long chain alkyl-containing compound, and thesaturated aliphatic bisamide are dissolved in the organic macromolecularresin which may contain the plasticizer, followed by the film formation.In the formed heat-sensitive recording layer, the organic low molecularsubstances exist in the form of particles dispersed in theabove-mentioned organic macromolecular resin, the particle size of thelow molecular substances being distributed in the range of about 0.5 to2 microns.

The thickness of the heat-sensitive recording layer is preferably in therange of 1 to 20 microns, more preferably in the range of 3 to 10microns. If the thickness is less than 1 micron, the heat-sensitiverecording layer insufficiently becomes opaque. If it is more than 20microns, thermal transmission in the recording or erasing of letterswith a thermal head becomes worse, which leads to bad sensitivity of theheat-sensitive recording layer to heat.

On the heat-sensitive recording medium of the present invention, thelayers which will be described hereinbelow can be provided.

For example, when recording and erasing images are carried out using athermal head, a protective layer comprising a thermoplastic orthermosetting resin, e.g., a polymethacrylate resin, a silicone resin,an acrylic resin, an alkyd resin, etc., or another resin, e.g., aphoto-curing or electron beam-curing urethane-acrylate resin, etc., as amajor ingredient may be provided on the heat-sensitive recording layerin order to improve the heat resistance of the heat-sensitive recordinglayer and to keep the compatibility with the thermal head.

For example, in order to prevent the migration of the organic lowmolecular substances contained in the heat-sensitive recording layerinto other layers, or to enhance the adhesion between the heat-sensitiverecording layer and the protective layer, an intermediate layer may beprovided between the heat-sensitive recording layer and the protectivelayer.

Moreover, a magnetic recording layer may be provided either on the faceof the support opposing the heat-sensitive recording layer, or betweenthe support and the heat-sensitive recording layer.

In the case where a magnetic recording layer is provided on the face ofthe support opposing the heat-sensitive recording layer, in order toprevent the wearing of the magnetic recording layer, a protective layercomprising a thermoplastic or thermosetting resin, e.g., apolymethacrylate resin, a silicone resin, an acrylic resin, an alkydresin, etc., or another resin, e.g., a photo-curing or electronbeam-curing urethane-acrylate resin, etc., as a major ingredient may beprovided on the magnetic recording layer.

The difference between the transparent state and the opaque state of thereversible heat-sensitive recording medium according to the presentinvention may be considered to be due to the difference of thecrystalline states of the organic low molecular substances dispersed inthe heat-sensitive recording layer.

To be specific, with regard to the transparent state, the followingexplanation can be made: The heat-sensitive recording layer in thereversible heat-sensitive recording medium is heated to a clarifyingtemperature T1-T2. At this time, the organic low molecular substancesdispersed in the heat-sensitive recording layer are melted, but notcompletely. Subsequently, when the layer is allowed to cool, theun-melted organic low molecular substances serve as cores and largecrystals grow therein in the course of cooling. Since the organic lowmolecular substances dispersed in the heat-sensitive recording layerexist as large crystals (single crystals), when a light is transmittedthereto, the degree of light-scattering on the interface thereof issmall, whereby the heat-sensitive recording layer can be assumed to bein a transparent state.

For the opaque state, the following description can be made. Theabove-mentioned heat-sensitive recording layer is heated to atemperature of higher than TW. At this time, the organic low molecularsubstances dispersed in the heat-sensitive recording layer arecompletely melted. When the layer is allowed to cool, because of theabsence of the core for crystal growth, crystals do not grow to a largesize in the course of cooling, but become an assembly of finemicrocrystals (polycrystals) after cooling. Consequently, a lighttransmitted in the heat-sensitive recording medium is scattered on theinterfaces of microcrystals, whereby this layer can be assumed to beopaque.

Utilizing such a reversible change, the reversible heat-sensitiverecording material of the present invention conducts recording anderasing images. According to the present invention, the clarifyingtemperature range can be extended.

The conventional reversible recording media have the problem in the factthat when the opaque lettered portion is erased by a heating mediumhaving insufficient heat capacity, such as a thermal head or a heatingmeans which cannot be heated for a sufficient period of time, theerasing ability is insufficient. This is considered to be due to thefact that the organic macromolecular resin in the heat-sensitive layerbecomes hard with a elapse of time due to the rearrangement of themolecule, whereby the heat energy required for becoming transparent isincreased to markedly decrease the sensitivity. In contrast, since aplasticizer is included in the case of the present invention, it may beconsidered that such a plasticizer plays a role in moderating theintermolecular force between the molecules of the organic macromolecularresin in the heat-sensitive recording layer to suppress the hardening ofthe organic macromolecular resin, thereby exhibiting an improvement inthe characteristics of erasing the above-mentioned white opaque, inaddition to the effect of extending the clarifying temperature range.

Since the long chain alkyl-containing compound having a melting point of50°-100° C. and the saturated aliphatic bisamide having a melting pointhigher than that of the former compound are used as the organic lowmolecular substances in the heat-sensitive recording layer of thepresent invention, even when the temperature reaches the level ofmelting the long chain alkyl-containing compound upon heating the layerfor becoming transparent, the saturated aliphatic bisamide remainsunmelted in the heat-sensitive recording layer. The remaining saturatedaliphatic bisamide serves as a core for crystals in the course of thecrystallization. Consequently, the upper limit T2 of the clarifyingtemperature is shifted toward a higher temperature, resulting in theclarifying temperature range being extended.

According to the present invention, the clarifying temperature range isextended by using the long chain alkyl-containing compound and thesaturated aliphatic bisamide having a melting point of not less than110° C. as the organic low molecular substance in a ratio of 98:2 to80:20. As a result, a reversible heat-sensitive recording medium capableof easily erasing images even with a heat medium having a small heatcapacity such as a thermal head can be provided.

EXAMPLES

The present invention will now be described by referring to the workingexamples. "Parts" means "parts by weight".

Example 1

A support was prepared by depositing aluminum on one face of atransparent polyethylene terephthalate film. The following coating forthe heat-sensitive recording layer was applied to the face of thesupport opposing the aluminum deposited face by means of a wire bar, anddried to form a 100 micron thick heat-sensitive recording layer.

    ______________________________________                                        Coating:                                                                      ______________________________________                                        Behenyl montanate      95 parts                                               Ethylenebis(behenic acid) amide                                                                       5 parts                                               Vinyl chloride-vinyl acetate copolymer                                                               300 parts                                              Tetrahydrofuran        1600 parts                                             ______________________________________                                    

Subsequently, the following coats were coated on the heat-sensitiverecording layer, and dried to form a protective layer of 3 micronthickness, giving a reversible heat-sensitive recording medium accordingto the present invention.

    ______________________________________                                        Silicone resin YR 3370    400 parts                                           (produced by Toshiba Silicone Co., Ltd)                                       Catalyst CR15              1 part                                             (produced by Toshiba Silicone Co., Ltd)                                       Isopropyl alcohol        1600 parts                                           ______________________________________                                    

Examples 2

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 1, except that N,N'-distearyldodecanedicarboxylicacid amide was used in place of ethylene-bis(behenic acid).

Example 3

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 1, except that N-stearyloleic acid amide was usedin place of behenyl montanate.

Example 4

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 1, except that distearylketone was used in place ofbehenyl montanate.

Example 5

A support was prepared by depositing aluminum on one face of atransparent polyethylene terephthalate film. The following coating forthe heat-sensitive recording layer were applied to the face of thesupport opposing the aluminum deposited face by means of a wire bar, anddried to form a 100 micron thick heat-sensitive recording layer.

    ______________________________________                                        Coating:                                                                      ______________________________________                                        Behenyl montanate      95 parts                                               Ethylenebis(behenic acid) amide                                                                       5 parts                                               Vinyl chloride-vinyl acetate copolymer                                                               300 parts                                              Di-2-ethylhexyl phthalate (plasticizer)                                                              20 parts                                               Tetrahydrofuran        1600 parts                                             ______________________________________                                    

Subsequently, the following coats were coated on the heat-sensitiverecording layer, and dried to form a protective layer of 3 micronthickness, giving a reversible heat-sensitive recording medium accordingto the present invention.

    ______________________________________                                        Silicone resin YR 3370    400 parts                                           (produced by Toshiba Silicone Co., Ltd)                                       Catalyst CR15              1 part                                             (produced by Toshiba Silicone Co., Ltd)                                       Isopropyl alcohol        1600 parts                                           ______________________________________                                    

Example 6

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 5, except that 10 parts of dibutyl adipate wereused in place of the plasticizer in Example 5.

Example 7

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 5, except that 10 parts of diisodecyl phthalatewere used in place of the plasticizer in Example 5.

Example 8

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 5, except that 10 parts of di-2-ethylhexyl sebacatewere used in place of the plasticizer in Example 5.

Example 9

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 5, except that a mixture of 150 parts of vinylchloride-vinyl acetate-vinyl alchol copolymer (SLEC-A: produced bySekisui Chemical Co., Ltd.) and 150 parts of polyester resin (VYLON 290:produced by Toyobo Co., Ltd.) was used as an organic macromolecularresin in the heat-sensitive recording layer.

Comparative Example 1

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 1, except that ethylene-bis(behenic acid) amide wasomitted and behenyl montanate was changed in an amount of 100 parts.

Comparative Example 2

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 1, except that ethylenebis(behenic acid) amide andbehenyl montanate were omitted and N-stearyloleic acid amide was changedin an amount of 100 parts.

Comparative Example 3

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 1, except that ethylenebis(behenic acid) amide andbehenyl montanate were omitted and distearylketone was changed in anamount of 100 parts.

Comparative Example 4

A reversible heat-sensitive recording medium was obtained in the samemanner as in Example 5, except that the plasticizer used in Example 5was changed in an amount of 80 parts.

The samples of reversible heat-sensitive recording media produced asdescribed above were thoroughly heated in an oven at 130° C., the wholesurface becoming completely white opaque. Thereafter, a heat block wasapplied on each of the samples at a pressure of 400 g/m² for 1 secondusing a heat gradient tester produced by Toyo Seiki Co., Ltd. to carryout recording and erasing tests. When a heat block at which thetemperature reached a temperature for allowing the substance to becometransparent was applied, the portion which was applied becametransparent. This was used to determine the clarifying temperaturerange. Also, using each of the reversible heat-sensitive recordingmedia, the white opaque portion thereof became transparent by an 8/mmthin type heat-sensitive head to evaluate the erasing characteristics.The results are shown in Table 1. The evaluation of the erasingcharacteristics was carried out by completely turning the entire surfaceof the reversible heat-sensitive recording media into a transparentstate in an oven, carrying out white opaque solid printing, and thenturning into a transparent state using the heat-sensitive head at whichthe maximum white opaque lettering density was reached. As a measure ofthe erasing characteristics, the ratio of erasing (R) was calculatedfrom the following equation:

    R=(B-C)/(A-C)×100(%)

wherein A, B, and C were the following reflection densities measured bya Macbeth reflection densitometer (RD-914):

A: the transparent skin density (the reflection density at which theentire surface becomes transparent in an oven)

B: the transparent density (the reflection density at a portion becomingtransparent using a heat-sensitive head)

C: the maximum white opaque lettering density

The criteria are as follows:

◯ . . . 80%R; Δ . . . 70%<R<80%; × . . . 70%>R

In Comparative Example 1, the difference (contrast) between the maximumwhite opaque lettering density and the transparent skin was, however,small and, thus, the image characteristics were too bad to evaluate therate of erasing (R). For this reason, the criterion was not assumed tobe practically applicable, and the rating of x was made without any realevaluation.

The clarifying temperature ranges: TW measured as described above areshown in Table 1.

                  TABLE 1                                                         ______________________________________                                                 Clarifying                                                           TW       Temperature                                                                              Erasing Characteristics                                   Ex.   (°C.)                                                                         (°C.)                                                                             A    B    C    R (%) Rating                           ______________________________________                                        1     30.0   77.5-107.5 1.50 1.32 0.40 84.5  ◯                    2     32.5   77.5-110.0 1.52 1.34 0.38 84.2  ◯                    3     25.0   67.5-92.5  1.52 1.31 0.42 80.9  ◯                    4     28.0   72.5-100.5 1.53 1.34 0.39 83.3  ◯                    5     35.0   72.5-107.5 1.60 1.50 0.70 88.9  ◯                    6     35.0   72.5-107.5 1.62 1.55 0.80 91.5  ◯                    7     30.0   65.0-95.0  1.65 1.45 0.58 81.3  ◯                    8     30.0   68.0-98.0  1.55 1.40 0.55 85.0  Δ                          9     32.0   70.5-102.5 1.30 1.27 0.65 95.4  ◯                    Com. 1                                                                              3.0    80.0-83.0  1.50 0.50 0.38 10.7  X                                Com. 2                                                                              2.5    67.5-70.0  1.50 0.40 0.35  4.4  X                                Com. 3                                                                              4.0    77.5-81.5  1.52 0.56 0.38 15.8  X                                Com. 4                                                                              35.0   70.0-105.0 1.65 --   1.14 --    X                                ______________________________________                                    

As is clear from the results of Table 1, the samples of the presentinvention to which a saturated aliphatic bisamide was added hadsignificantly extended clarifying temperature range in comparison withthe samples of Comparative Examples 1 to 3, which added no suchcompound, and could be confirmed to have an enhanced erasing rate.Examples 5 to 9 made it clear that the clarifying temperature range andthe erasing rate can be further improved by the addition of aplasticizer. It can be confirmed from Comparative Example 4 that theamount of the plasticizer formulated has a great influence on thecontrast (visuality) between the maximum white opaque lettering densityand the transparent skin.

What is claimed is:
 1. A reversible heat-sensitive recording mediumhaving a heat-sensitive recording layer capable of reversibly changingthe transparency thereof with organic low molecular substances dispersedin an organic macromolecular resin provided on a support, at least onelong chain alkyl-containing compound having a melting point of 50°-100°C. and at least one saturated aliphatic bisamide having a melting pointof not less than 110° C. being used as the organic low molecularsubstance in a ratio of 98:2 to 80:20.
 2. The reversible heat-sensitiverecording medium as claimed in claim 1, wherein the macromolecular resincontains a plasticizer in a ratio of 1 to 10% by weight, based on thetotal solid of the heat-sensitive recording layer.
 3. The reversibleheat-sensitive recording medium as claimed in claim 1, wherein an esterformed from a higher fatty acid and a higher alcohol is used as saidlong chain alkyl-containing compound.
 4. The reversible heat-sensitiverecording medium as claimed in claim 1, wherein a higher aliphatic amideformed from a long chain saturated aliphatic acid and a alkaline diamineis used as said saturated aliphatic bisamide and a wax, ester, amide orketone is used as said long chain alkyl-containing compound.
 5. Thereversible heat-sensitive recording medium as claimed in claim 1,wherein said saturated aliphatic bisamide has a melting point of 130° to150° C.